Multilayer, plastic pipe and its method of manufacture are long and well known. Original plastic pipe, multilayer or otherwise, was typically constructed from a polyolefin or substituted polyolefin, e.g., polyvinyl chloride (PVC), and it was typically smooth-walled (both inner and outer walls), extruded from annular dies, and exhibited poor diametral compressive strength, i.e., it was easy to collapse by the application of a compressive force.
The problem of poor compressive strength has been addressed on a number of different levels. One solution was to increase the wall thickness of the pipe, but this came at the cost of using more polymer to construct the pipe, an increase in pipe weight, and a loss of pipe flexibility. Another solution was to add corrugation to the outer wall of the pipe, e.g., U.S. Pat. Nos. 3,538,209 and 3,677,676. Still another solution was to use a three-layer pipe construction in which the middle or center layer was foamed, e.g., U.S. Pat. No. 6,176,269. Yet another solution was to use a polymer that exhibited a greater impact strength or resistance than the conventional polymers used for these applications, e.g., U.S. Pat. No. 6,841,620. Improvements on these last two approaches remain of interest to the pipe industry.
WO 2007/017166 describes a pipe of polymer micro-foam material having an impact strength value measured at 0° C. which is higher than 100% and up to 400% of the impact strength as required in the EN-norm of a pipe of the same dimensions and basic material in non-foamed condition. It also describes methods for producing such pipe.
WO 2001/005569 describes a method for forming an article comprising closed-cell micro-foam from thermoplastic in which at least one molten thermoplastic comprising a foaming agent is subjected under pressure to a forming operation and, after the pressure has been at least partially released, is cooled, characterized in that the amount of foaming agent is substantially identical to the amount corresponding to that quantity of gas released by the foaming agent. The micro-foam has a close-packed structure of foam cells with a specific foam-cell diameter, substantially uniform throughout the foam, at the pressure prevailing during cool-down.
WO 2000/015405 describes the extrusion of foamed articles made from thermoplastic. A molten mass comprising heated, pressurized plastic which has been mixed with a foaming agent is pressed firstly through an orifice which shapes the article, then through a nucleator, i.e., a perforated plate, and then is cooled to form a very fine micro-foam structure. Particularly good results are obtained if the molten material is compressed without exposure to any significant shear force for a short time immediately after it leaves the nucleator.
U.S. Pat. No. 6,684,910 describes a pipe of layers of thermoplastic polyolefins of the same type but with different properties. The layers are connected inseparably with one another, an outer first layer and a second layer that adjoins the first layer on the inside and consisting of a material that has not been cross-linked. The second layer has a full notch creep test (FNCT) of approximately the same value or the notched pipe test (NPT) value of a one-layer standard pressure pipe. The first layer has an FNCT or NPT value that is higher by a factor of at least 3 than that of the second value. A third layer is provided that adjoins the second layer on the inside, and it too consists of material that is not cross-linked and has a FNCT value that is higher by a factor of at least 3 than that of the second layer and an NPT layer that is higher by a factor of at least 2 than that of the second layer.